Doppler ultrasound has been used to measure blood flow velocity for many years. The well-known Doppler shift phenomenon provides that ultrasonic signals reflected from moving targets will have a shift in frequency directly proportional to the target velocity component parallel to the direction of the ultrasound beam. The frequency shift is the same for any object moving at a given velocity, whereas the amplitude of the detected signal is a function of the acoustic reflectivity of the moving object reflecting the ultrasound. Pulse Doppler ultrasound systems commonly produce a spectrogram of the detected return signal frequency (i.e., velocity) as a function of time in a particular sample volume, with the spectrogram being used by a physician to determine blood flow characteristics of a patient.
Typically, a user of ultrasound equipment finds it rather difficult to properly orient and position an ultrasound transducer or probe on the patient, as well as to select a depth along the ultrasound beam corresponding to the desired location where blood flow is to be monitored. This is particularly true in ultrasound applications such as transcranial Doppler imaging (TCD). The blood vessels most commonly observed with TCD are the middle, anterior, and posterior cerebral arteries, and the vertebral and basilar arteries. The Doppler transducer must be positioned so the ultrasound beam passes through the skull via the temporal windows for the cerebral arteries, and via the foramen magnum for the vertebral and basilar arteries. The user of the ultrasound equipment may find it difficult to locate these particular windows or to properly orient the ultrasound probe once the particular window is found.
A complicating factor in locating the ultrasound window is determination of the proper depth at which the desired blood flow is located. Commonly, the user does not know if he is looking in the correct direction at the wrong depth, the wrong direction at the right depth, or whether the ultrasound window is too poor for appreciating blood flow at all. Proper location and orientation of the Doppler ultrasound probe, and the proper setting of depth parameters, is typically by trial and error. Not only does this make the use of Doppler ultrasound equipment quite inconvenient and difficult, it also creates a risk that the desired sample volume may not be properly located, with the corresponding diagnosis then being untenable or potentially improper.
Once blood flow has been located, it is usually scanned along the course of the vasculature to determine if there are any localized regions in which there are flow abnormalities, which may indicate various diseases. The spectrogram is typically observed for hemodynamic clues indicating disease. However, in conventional Doppler ultrasound systems, regions having abnormal flow may be displayed ambiguously. For example, in some cases, jagged black regions, which may be construed as regions of no detected blood flow, may appear in regions where actual blood flow is indeed present. Additionally, blood flow information for regions having hemodynamic parameters of interest may be displayed in a spectrogram with aliased spectral velocities and with high-amplitude, low velocity clutter signals. The result is a spectrogram indicating blood flow velocities that “wrap around” through a maximum velocity to appear as a negative velocity along velocity axis. Both the aliased velocities and the clutter signals can severely compromise detection of peak blood flow velocity and other hemodynamic parameters.
The previously described issues with conventional Doppler ultrasound systems are often due to artifacts resulting from Doppler signal processing. A possible cause of artifacts is the presence of a bruit signal that often accompanies the pathological condition of vasospasm, a condition that results in a constriction of the vessel lumen and results in high velocity blood flow.
A bruit is a signal that appears on a Doppler spectrogram due to periodic tissue motion having a frequency in the audio range and an excursion distance of less than a wavelength of the ultrasound. In the case of a Doppler carrier frequency of 2 MHz, the wavelength is less than 780 μm. A bruit can easily be much larger in amplitude than the blood flow also present in the Doppler sample volume. For example, the detected power in a bruit signal can easily exceed that in the blood flow by 30 dB. Moreover, a bruit can be accompanied by harmonics that fall off quickly in amplitude, and by definition, bruit signals lack a directional component. The bruit is also generally significantly lower in its Doppler shift than the associated blood flow. These characteristics of bruits imply that the mean velocity estimate for the motion in the Doppler sample volume can be severely biased downward. In conventional Doppler ultrasound systems, the downward biasing will cause black regions to be displayed in regions where normal blood flow is detected since signals that have associated velocity below a clutter threshold are automatically colored black. One remedy for bruit signals is to calculate mean velocity in the spectral domain and exclude the low velocity territory where bruits tend to be present. This approach however is time consuming in that it requires a Fourier transform to be computed at every analyzed depth
Another potential cause of artifacts is high velocity aliasing due to the Doppler shift frequency of detected blood flow exceeding the Nyquist frequency of the Doppler ultrasound system, the result of which is to bias the detected mean velocity to zero. The biasing is potentially significant in that the high velocities in excess of the Nyquist sampling limit are interpreted as high velocities in the opposite direction of the true blood flow and act to negate any high velocity signal data in the true flow direction. Such aliasing is can be remedied by increasing the Doppler pulse repetition frequency (PRF). However, in conventional Doppler ultrasound systems, increasing the Doppler PRF comes with a tradeoff of reducing the maximum interrogation depth, which is limited by the round trip distance an ultrasound pulse can travel before a subsequent ultrasound pulse is launched by the system.
Therefore, there is a need for an Doppler ultrasound system and Doppler signal processing method for displaying regions of blood flow having a variety of possible hemodynamic parameters and indices of interest in a fashion that yields unambiguous understanding of these parameters and where they spatially arise.